The present invention relates to a rewritable thermal recording medium and a recording method by which information can be repetitively recorded and erased by controlling the heat treatment conditions.
Generally, output information from an OA apparatus is printed as a hard copy on a sheet of paper by a printer or displayed as an image formed by a display device. With the recent advance of OA, the amount of data to be processed keeps on increasing accordingly. Therefore, when information is output on sheets of paper, a large amount of sheets must be used, and this is a problem in protecting resources. Also, outputting information to a display device represented by a CRT or an LCD poses the problem of consumption of a large amount of power.
For these reasons, a display means capable of displaying an image with high visibility, capable of repetitively recording and erasing the image, and not requiring energy to hold the image, i.e., a rewritable recording medium, is expected in various fields.
A solid or semisolid recording material is used in a rewritable recording medium. Jpn. Pat. Appln. KOKAI Publication Nos. 55-154198 and 57-82086 have disclosed a rewritable recording medium using a recording material consisting of a low-molecular organic compound and a high-molecular resin matrix. When this recording medium is used, images can be recorded and erased by using a thermal printer head (to be referred to as a TPH hereinafter). Also, this recording medium is well balanced in the above-mentioned characteristics required of a rewritable recording medium. Therefore, the recording medium is beginning to be used as prepaid cards and the like.
Unfortunately, the visibility of a rewritable recording medium using the above recording material is unsatisfactory because recording and erasure of an image are performed by reversibly changing the recording material between an opaque state and a transparent state. Also, this rewritable recording medium itself cannot display color pictures or patterns.
Jpn. Pat. Appln. KOKAI Publication No. 4-50290 has disclosed a composition consisting of a leuco dye and a developer including an acidic group having a developing function and a basic group having a decolorizing function. When a predetermined thermal energy is supplied to this composition, a chemical change can be repetitively caused between a colored state and a decolorized state.
Jpn. Pat. Appln. KOKAI Publication Nos. 4-247985, 4-308790, and 4-344287 have reported compositions prepared by mixing a leuco dye and long-chain phosphonic acid as a developer. When thermal energy is applied, these compositions change their crystal structures and can reversibly change between a colored state and a decolorized state.
Any of the above recording materials is colorless in equilibrium and colors when set in a quasi-equilibrium state by application of a predetermined amount of heat. Accordingly, a rewritable recording medium using any of the above recording materials can perform positive recording and is suited to record characters on a colorless recording surface by using a TPH.
To display, however, an image including a large number of colors by rewritable recording media using only the positive recording materials described above as recording materials, it is necessary to form a recording surface with a predetermined pattern by using recording materials which can take on these colors and to set the recording materials forming the recording surface in a quasi-equilibrium state. For this reason, the above rewritable recording media are not suited to display an image including a large number of colors. Additionally, when such rewritable recording media are used, it is difficult to further record characters or the like in an image in a colored state. As described above, complicated color images are difficult to display by the conventional rewritable recording media.